Think I might just sacrifice all my non-heroic and non-mythical souls.
Finished the actual rewards part about 10 mins after the event started. That said I'm collecting as many Sentinel mythical soul fragments as I can, as the only Sentinel character that I don't have in the Mythical souls is Icarus (shows as a 1 in 7 chance?). Unless of course, I get one out of the souls I'm redeeming in the book for sacrifice.
I currently have all the Magus and Warrior characters available in those Mythical souls, so might just hold onto them till something new appears, so I have a good chance of getting that.